it loses its green color, becoming colorless, with a few
reddish brown specks scattered through it (_F_).

In some species the sexual organs are borne directly on the filament
(Fig. 21, _G_).

Large zooespores are formed in some of the green felts (Fig. 22,
_A_), and are produced singly in the ends of branches that become
swollen, dark green, and filled with very dense protoplasm. This end
becomes separated by a wall from the rest of the branch, the end
opens, and the contents escape as a very large zooespore, covered
with numerous short cilia (_A_ ii). After a short period of
activity, this loses its cilia, develops a wall, and begins to grow
(III, IV). Other species (_B_) produce similar spores, which,
however, are not motile, and remain within the mother cell until
they are set free by the decay of its wall.

ORDER V.--_Characeae_.

The _Characeae_, or stone-worts, as some of them are called, are so
very different from the other green algae that it is highly probable
that they should be separated from them.

The type of the order is the genus _Chara_ (Fig. 23), called
stone-worts from the coating of carbonate of lime found in most of
them, giving them a harsh, stony texture. Several species are common
growing upon the bottom of ponds and slow streams, and range in size
from a few centimetres to a metre or more in height.

The plant (Fig. 23, _A_) consists of a central jointed axis with
circles of leaves at each joint or node. The distance between the
nodes (internodes) may in the larger species reach a length of several
centimetres. The leaves are slender, cylindrical structures, and like
the stem divided into nodes and internodes, and have at the nodes
delicate leaflets.

At each joint of the leaf, in fruiting specimens, attached to the
inner side, are borne two small, roundish bodies, in the commoner
species of a reddish color (Fig. 23, _A_, _r_). The lower of the two
is globular, and bright scarlet in color; the other, more oval and
duller.

Examined with a lens the main axis presents a striated appearance. The
whole plant is harsh to the touch and brittle, owing to the limy
coating. It is fastened to the ground by fine, colorless hairs, or
rootlets.

[Illustration: FIG. 23.--_A_, plant of a stone-wort (_Chara_),
one-half natural size. _r_, reproductive organs. _B_, longitudinal
section through the apex. _S_, apical cell. _x_, nodes. _y_,
internodes. _C_, a young leaf. _D_,